How will multiple GPUs improve Cycles' rendering performance?
There are lots of good articles out there on this topic, and short of recreating them, I can say that the two cards I use are not in SLI (wrong motherboard for it) and it uses them like two separate cores. It'll render two chunks at a time, if you tell it to use both. Some of the articles say that running in SLI is a little slower than running two separate cards, but I'm not familiar with the principles behind that.

I don't know if the rendered preview (in the viewport) benefits at all, but it can't hurt.